Frequency modulation receiver tuning aid



Dec. 2.1, 1948. B. s. vxLKoMr-:RSON 2,457,016

FREQUENCY MCDULATION RECEIVER TUNING AID Filed; 0G12. 25a 1945 2 Sheets-Sheet l B. s. VILKOMERSON 2,457,016

FREQUENCY MODULATION RECEIVER TUNING AID 2 Sheets-Sheet 2 Snuentor EENJAMm SVILKUMEBSDN Comeg Patented Dec. 21, 1948 UN il? STATES;

FREQUENCY MODULATION RECEIVER.

TUNING All) Benjamin S. Vilkomerson, Camdemn-Nl-leassignor to Radio Corporation of America,.-a :corpofraeiA tion of Delaware Application 0ctoberf26', 1945;5SeirialfNSiGZdg'lSS (Cl. Z50-4o.)

9 Claims. l

My present yinvention relates generally to tun-l ing aids for frequency modulation receivers. and

more particularly to a novel and improved device for enabling a listener accurately toftunehis receiver.

One of the main objects of this invention is to.y

provide a novel arrangement for visually indicating inf a :frequency modulatio'nreceiver of the locked-in oscillator type vwhether or not frequency modulation (FM) signals are being re'- ceived, and whether or not the receiver is ac*- curately tuned to resonance with the center, or midnchannel, frequencyof the desired FM station. In accordance with yone aspect of theinvenr tion there is provided a visual indicator vtube of.

the dual shadow type. `One of the shadows has' its angle Yor area controlled in accordance with the frequency of the locked-in oscillations, while the Y.

second shadow has its angle controlled .ir1re-l sponse to the magnitude of the' received FM sigf In this way, observation of the dual shadow nal. informs the listener whether FM signals 'are be-- ing received, and whether the receiver is accurately in tune.

Another important object of my present invention is to provide a novel method of preventing Y spurious detection alongthe slopes of the' FM'r detection 'characteristic at frequencies below '01 above' the peakl response afrequencies of lthe de'-4 Such method vpositively prevents the impression upon the detectorV input?I terminals of any frequency-Variable energy outtection characteristic.

side of a restricted frequency range, such restricted frequency range beingsubstantiallyless' thanthe'frequency difference between the' peak response frequencies of the' detector characteristic. spot tuning.` system `for receiving FM signals wherein a frequency `change-responsive detec tor with opposite peak output response frequen;

My invention further provides a one-` cies centered about the FM" .carrier frequency is# preceded by a lockedn oscillator, or other network, of such frequency response characteristic that its pass'bandis centered*on-saidFMfcarrier freuuency and/.is I'completely withimiand of ineg- Another object" of this invention is to :providef in-an FM receiver'ofsthe typefembodying a locked-1 in-oscillator; whose lockeinrange lis 'substantially' 45 ligibleamplitude at v.and beyond the aforemcn--A- tioned opposite detector. peak response1frequen-- lessthan fthe frequency spacing .between fthel peak frequencies 'of'rthe-FM detectorga tuningzgindi--- catortube .which 'hasxa pair. of electronicsha'dowsf both-1. shadows being: :of: minimum :angle or area when.` the AFM signals. impressed onf the Vlocked-.ini oscillator .have a center rfrequency. lequalftoe the predetermined :operati-ngi.` frequency of suchuosi cillator;rv

Still other objects offmynventionrare toim.- proveA generally thewefliciencywand simplicity-of; tuning indicatorsforrFM'receivers and more'es-.ffl

peciallyfito provide tuning aids .infsuch receivers in an` economica'lf'manneri* Other objects andsfeaturesrloh.my invention fr will best be understood by. reference-tol. the -fo1.- lowing description, taken infconnection withuthe-V drawings, in which I havefA-indicated-,za l"circuit whereby my invention? may-...ba carried-.intoei-.vY` fect.

Ini'fthe drawing 1.

Fig. 1 is a circuit, diagram of yan. embodimentof my invention; i

Figs. 2a, 2b,gand :2c show respective-:appearances -of lthe' indicator tar-get. for differenti-condi.-

tions of reception;`

Fig. 3 shows atypical FM detection -characf teristic; and

Fgffl isa circuit-diagram of;v a A'modifica-tion of my' invention;l

In-Fig;` 1 I have shoWnra receiver-scircuit'em; v bodying myfinvention-and including a=-locke`d-in oscillatoracting as a yfrequency.,dividerf'of signals. Suchcircuitis generaily *ofr the type dis-' closed'zby- GrJL.V `Beersf-finrhis U. S.f-Patent NonV 2,356,201, granted August 22, 1944. More VAspeci--m iically; the locked-:in oscillator *circuit shown fis ondary. ycircuitA-V l5 :each'zitunedi to'fthetgoperating intermediatefrequency (LF.) of .the system." I

but mayfrusejl :any vother suitable .1 intermediate frequencyn' The received FM wavesmayxbe thoserwhich' are vtransmitted inthe: .presentfassigned :FMcband of: 42 150150` -megacycles (m'cz, .or .they'.may be in-A there'cently. assigned" 88` tto- 108.-"mc:.:-range,fr 'Ihezty l inventionris anotfi-lixniteiztT tozfanyrparticularefreei- 3 quency band or to the reception of FM waves, since it is generally applicable to angle modulated carrier waves. Those skilled in the art of radio communication are fully aware of the fact that in each channel the FM waves transmitted in the aforementioned assigned FM band are presently allotted a maximum frequency swing. of 150 kc. (kilocycles). That is, a maximum deviation of i 75 kc. in respect to the mean, or center, frequency. The frequency deviation at any instant is dependent upon the amplitude of the modulation signals at the transmitter, while the time for a cycle of the frequency deviation is equal to the period of the modulating signal per se. It is to be understood that my invention is not limited to the specific locked-in oscillators shown in the Beers patent and Corrington application.

The collected F'M Waves are usually selected in one or more stages of tunable radio frequency amplification, after which they are combined with locally-produced oscillations at the first detector network. The output of the first detector, or converter, is the I. F. (intermediate frequency) energy.A In other words, the LF. energy is the FM wave whose mean frequency has been reduced to a much lower frequency, but whose frequency deviation is unchanged. After amplification by one or more stages of I. F. amplification, the I. F. energy is applied to the locked-in oscillator for concurrent frequency division and reduction of the frequency deviation'.-

The FM signal energy, with its mean frequency at the roperating I. F. value, is applied to the input grid 2 of tube l. The latter may be, for example, a miniature tube of the pentagrid type. The input grid 2 is connected tothe high alternating potential side of the secondary 'circuit l5. The low potential side of circuit I5 is returned to the grounded cathode 3 through resistor R1 shunted by condenser C9, the latter comprising a network designated by the numeral I6. The function of the network I6 is to provide voltage across the resistor element R1, in response to grid current flow through the signal input grid circuit. Such grid voltage developed across the resistor R1 may be utilized for automatic volume control (AVC). The vAVC voltage is employed automatically to bias the control grids of the preceding amplifier tubes in a manner well-known to those skilled in the art. l

The plate circuit 5 in Fig. 1 has an operating band width of between 50 and 65 kc. Awhile the mean frequency of the output voltage of the locked-in oscillator has a value of one-fourth of 10.7 mc., i. e. 2575 kc. This is appropriate since the effect of the locked-in oscillator network has been to divide the mean frequency of the FM wave energy by a factor of 4, and the overall frequency deviation range has, also, been divided by the same factor. lThe pass band width should be sumciently above 37.5 kc. to provide suitable operating tolerances. Further, the `plate current is substantially constant regardless'fof ramplitude variations at the signal input grid.

The advantages of frequency division atthis point of the FM receiving system have been explained in the Beers patent. The extension cf the lock-in range of the oscillator accomplished by the circuit employed herein, and shown in the aforesaid Corrington application, enables reception of waves which are frequently modulated over a wider range, and guards against distortion which Amight otherwise occur by reason of breakout. enact. .since thez wave energy developed across the plate circuit 5 is modulated in strict accordance with the originally received FM signals save that the mean frequency and extent of frequency deviation have been proportionately reduced, the output of. the locked-in oscillator may now be subjected to suitable frequency discrimination and rectification to repro-duce the signals.

Further considering the specific construction and operation of the locked-in oscillator circuit, the oscillator grid 5 of tube i is connected by lead 5 to the high alternating potential side of tickler coil L1. The lower end of the coil is connected to ground through the parallel-connected combination of resistor Re and condenser Cm. Condenser C2 shunts the tickler coil L1. The grid circuit 5 may be resonant to any frequency within a considerable range, and the capacity of C2 is therefore not critical. In a practical circuit, the resonant frequency of L1C2 can be of the order of double the frequency of the tank circuit 5.

The screen grid 2 of tube l is connected to a source of positive potential +S, say +80 volts,

and is bypassed to ground for I. F. curents by con-- denser C1. The plate t of tube l is connected to the +B terminal, say at volts, through the.

resonant circuit 5. The resonant tank circuit 5, tuned in the present illustration of my invention to 2675 kc, consists of the parallel-connected combination of coil Lz, resistor R4 and condenser C4. The resistor element may be made variable, as indicated by the arrow. Coils L1 and L2 are magnetically coupled to provide the proper amount of regenerative feedback in order continuously to produce oscillations at the desired frequency division ratio. Condenser C5 bypasses the low end of resistor R4 to ground for the oscillator current.

The auxiliary resonant network, concerning which reference may be made to the Corrington application for a more complete description, is designated 5, and consists of the parallel-connected combination of condenser C3, resistor Re and coil L3. The coil L3 is magnetically coupled to the plate coil L2, and this coupling is indicated by the dashed arrow through both coils. The resistor R2 ispreferably variable, as indicated. The degree of coupling between coils L3 and Lz predominantly determines the extent of the lockin range of the oscillator. There are other factors which influence the lock-in range, but these factors are minor compared to the coupling be tween the aforesaid coils.

The auxiliary resonant network5" is tuned to the same resonant frequency as the plate circuit 5. For maximum lock-in range the auxiliary coupled circuit 5 is slightly overcoupled (closer than critical coupling) with respect to tank circuit 5. It has been found possibie to secure a lock-in range, through all prior radio frequency and I. F. selectivity, of i kc. or more .in terms of the input to the locked-in oscillator.

Turning now to the FM detector D, I utilize a discriminator circuit, which may be of suitable known type, e. g., that shown in U. S. patent to S. W. Seeley, No. 2,121,103, granted June 21, 1938, or to J. D. Reid, 2,341,240 granted February 8, 1944. The high potential side of plate circuit 5 is coupled to the detector by condenser Cs. In Fig. 3 I have depicted a typical detection characteristic for the detector schematically shown in Fig. 1. The frequencies F1 and F2 denote the detector input frequencies which give opposite peak direct current response. The resultant-instantaneous output voltage of the `detector `-is zero at the frequencyFc. It is highlyzdesirable, ofcourse, that .the .curvebe highly l'inear fbetween the peak lresponse, or foutput frequencies. I'The detector .circuit used iin Fig. v1 should .possess such acharlacteristic.

'From Fig. .3 it can be seen that there fis adis- Itinot 4advantage in fusing a locked-'in oscillator 1to feed ran FM detector of the sloping 4iilter type so Sfar .as prevention of l-so-called three-spot tuxringiislccncerned. .As is well known FM detection-can occur, iin the :absence of a signal-swing restraining device, on the outer slopes of fthe peaks F1 :and F2, i. e., at a frequency below F1 or labove `Such detection produces distortion in t.the audio output system F, and is also very conlfusing to the listener in tuning an vFM receiver. `In my arrangement, the locked-in oscillator sysvtem functions positively to maintain the signal deviatio-ns with a range of approximately 125 to $32.5 kc. indicated `by points .r Aand y, Fig. 3, of the detection characteristic, thereby automati- .cally `providing single spot tuning. With such an overall lock-in range '(91, y) of 50 to 65 kc., vthe vmaximum deviations ofthe FM vsignal applied to detector D cannot reach the response peaks F1 vand F2 which I prefer to have 100 to 130 kc. apart.

A restricted lock-in range prevents detection beyond the peaks F1 and F2, because the locked-in oscillator will follow the frequency deviations of the I. F. signal energy only as long as they are within the -lock-in range. When the I F. voltage applied lto the -locked-in oscillator is at some -frequency below or above the lock-'in range, the oscillator, instead of following that frequency, de- -velops break-out. That is, the oscillator snaps back to nearly its own center frequency. :('Ihere is some 'shift of 'the 'center frequency even inthe absence of lock-in.) The FM detector vgets its alternating current energy only from the lockedin oscillator. Since the locked-in oscillator supplies output energy either at its center frequency or .at frequencies corresponding to onefourth of 'the I. F. yvaluewithin its :lock-in range,

all `of the .alternating current energy applied `to the F. M. detector will fall well within the mequency range vincluded :between F1 and F2. When the receiver is detuned so that the vI. F. energy is of such frequency 'that one-fourth of it falls beyond points F1 fand F2, the alternating current `energy fed to the detector will be at the center frequencyof the .locked-in oscillator, since the oscillator does tnot lock-'in with I. F. energy so far removed from the normal operating value.

It maybe pointed out that a network or vpassband 4G having indefinite impedance rejection characteristics such as to give zero response Vat frequencies F1 and F2 may be used,.in place of the locked-in oscillator, between network I3 and the detector D as shown in Fig. 4. Here, again, the detector output would be kzero below and above the lower and yupper response peaksF1 and 'F2 respectively. For example, there may :be substituted for boi: im of Fig. 4 the infinite impedance rejection network described by G. Mountjoys'in U. S. Patent No. `2,137,475, grantedNovember-ZZ, 1938. Undesirable spurious responses beyond peaks vF1 and F2 are, accordingly, readily prevented by restraining the signal -deviation at the detector input terminals to prescribed limits '(as :c and y) well within the peaks F1 and F2.

In an 'FM set, it is further desirable to indicate visually whether the receiver is `tuned to one side or 'not signals vare actually.beinghreceived. These conditions are readily visually indicated by my invention. In general, I utilize AVC voltage to .indicate lwhether lor not TFM signals fare being received, awhile I employ the frequency of the flocked-inoscillator outputenergy to indicate the condition of vresonance of the receiver. By observing both .indications the receiver operator is enabled to tune the receiver to the desirable midvfrequency of the station channel.

yThe :indications :are provided by an Aelectronic .indicatcrtube of the dual shadow type. Forexample, v:theremay Vbe used the 6AF5 type of tube. 'I ,have'designa'ted the indicator -tube by reference numeral .20. The ltube 253 is schematically represented, since Isuch tubes `are well-known to those :skilled in .the art of radio communication. The etube :2li comprises the electron emission element, -or cathode, `ZH which .may be returned to the `ground -;=B iterinina'liof `the direct current source (not shown)- The spaced electron ray control rods, .nor electrodes, A and B control the .flowof ,electrons to respective adjacent areas of the sflu crescent target 22.

l'Target 22 isusually an inverted frustum of a icone, zthefflared-iendbeing the viewing end. The .inner face .of the target .is :coated vwith fluorescent material kwhich .glows in response to electronic :bombardment from cathode 2l. The rods A and eB fare usually located on opposite sides :of the cathodefand in -alignment'therewith whereby they may cast electronic shadows on the target. Thatis, lthe electrodes A and B are electron deflection rods.

The target 22 is connected to the +B terminal (say at +250 volts) While vrods A and B are energized fromldilferent points of lthe receiver systeni. Rod A :is connected to the high potential side of a resonant circuit which is sharply .tuned to the :mean operating frequency of the locked-in oscillator. In other words, circuit 23 lwithfan of 10.7 rncnand a frequency division V'of 4 -is sharply tuned 'to a frequency of A2675 kc. The circuit '23 -is magnetically and loosely cou- ,pledtoiplate circuit 5. The low potential side of circuit "23 is connected preferably to a positive Vvoltage :point which may be chosen from a range o-fizerotoapproximately +40 volts. It can, thereriore, .be :seen that norinallythe rod A is at a positive :potential with respect to the grounded cathode 221.

:The rod B fis Avaried in voltage in response to 'variations Yin 'magnitude of the AVC voltage developed across resistor R1. As shown, the rod B is :connected to 'the plate 3@ of the direct cur- 'rentvoltage amplier tube 3l. Tube 3l may be a tube Aof the v6.15 type, and has its cathode 32 grounded. Plate 3l] is connected to the +B termi- :nal of the .direct current source through a resistor Accordingly, when lthe space current g :0f itu-be 31 is at maximum the Voltage drop across .resistor mais at maximum, and, therefore, the .rod Bwillbfe at 4minimun-l positive .potential with respect ...tothe highly positive target 2v2.

The space :current :flow of tube .Si regulated by control grid il which is connected through 4thealternating .current filter resistor '35 and lead `523.6 `to the :un-grounded end cf resistor R1. The .bypass-'condenser 35 may be inserted to bypass alternating fcurrent components from the lead '36. It 4will now be seen that when FM lsignals fexistin circuit t5 thegrid current flowing through :resistorR1wilhdevelop a voltage which is neg- 'ative with frespect to ground. This `negative voltfageis .applied to grid ,34 with the result that Ythe space current flowing through tube @l -is diminished, thus causing the voltage drop across resistor 33 to decrease. This means that rod B approaches in potential the voltage of target 22. Hence, rod B will deflect less and less electrons from the adjacent target area as the FM signal magnitude at circuit l approaches a maximum value. Conversely, when there is no FM signal at circuit lli the grid 34 will have the same potential as the grounded cathode 32, and the rod B will have minimum positive potential relative to the highly positive target 22. This means that the rod B will deflect sufficient electrons to cause a shadow area on the adjacent surface of` the target which is at maximum angular width.

Se as the action of rod A is concerned, that electrode will have minimum deection effect on the electrons flowing in its vicinity towards the target either in the absence of signals or when the receiver is accurately tuned to midchannel frequency, since in either of such cases, the locked-in oscillator delivers maximum output at the frequency to which circuit '23 is tuned. By choosing the correct value of positive bias ior rod A, the shadow angle caused by rod A can be made to adjust to zero at the peak response of the resonant pickup circuit 23, which corresponds to correct tuning. That is, in the latter condition the local oscillator of the receiver is heterodyning the received FM carrier to the middle of the I. F. channel and the operating center frequency of the lock-in range of the locked-in oscillator. Positive bias applied to rod A, also, reduces the amount of radio frequency power required from the pickup circuit coil to reduce the corresponding shadow angle to zero.

IThis permits looser coupling between circuit 23 and circuit 5, and, hence, provides sharper tuning to peak response.

In Figs. 2a, 2b and 2c I have depicted the relative appearances of the shadow angles A and B for different voltage conditions of the deflection voltages A and B respectively. Thus, each of these figures shows the flared end of the fluorescent target 22. rIhe shadow angles A and B are produced by the respective rods A and B as the eiective voltages thereof vary. Fig. 2a shows the shadow angle B at maximum width when no FM signal is being received, While the shadow angle A is of minimum or Zero width, because the locked-in oscillator section is producing lockedin oscillations at the center frequency Fe. This means that maximum radio frequency voltage is applied to rod A thereby causing minimum electron deflection. Minimum electron deflection results from the fact that the rod A is eiectively at maximum positive potential with respect to the target 22.

Referring to Fig. 2b the shadow angle A has widened out, whereas shadow angle B is diminished. This shows the condition for reception of FM signals with the receiver partially detuned. In that condition of ofi-resonance, the FM signal magnitude at circuit l5 is not at maximum. I-fence, the shadow angle B will have an inter mediate value, while the shadow angle A will be enlarged. This is due to the fact that the oscillator tube i is locked to a center frequency differing from the resonant frequency resonant circuit 23. I-Ience, rod A will have reduced positive peak potentials with respect to cathode 2l. The magnitude of the oscillations provided by the locked-in oscillator are not significantly less when the receiver is off-tune. In fact, they might be slightly more, or slightly less, ldepending on whether the oscillator is being accelerated or decelerated by the I. F. energy. What actuates ray control electrode A is resonant circuit 23, which, being sharply tuned to the oscillator center frequency, applies the maximum voltage to rod A when the oscillator is operating in the center of its lock-in range. The said voltage drops off rapidly in magnitude at frequencies removed above or below the oscillator center frequency (say by ilO kc.) due to the peaked amplitudefrequency response characteristicof resonant circuit 23. It will be noted that this occurs even though the oscillator voltage remains about the same.

In Fig. 2c both shadow areas A and B are substantially zero thereby indicating the irl-tune condition of the locked-in oscillator, and maximum FM signal being applied -to circuit l5. It will, therefore, b-e seen that tuning any desired station becomes a question of adjusting the station selector c-ontrol until both shadow angles A and B are of minimum width, as shown in Fig. 2c.

It is to be clearly understood that the tube 3l need not be constrolled from the grid circuit of the locked-in oscillator tube l, but, in general, the grid :it may be varied in voltage from any point in the receiving system which develops negative voltage whose magnitude is proportional to the mag.- nitude of the received FM signals. Furthermore, where large variations in AVC voltage are encountered the tube 3l is preferably one of the re mote cut-off type. In conjunction with the AVC voltage indication B', shadow angle A may be replaced by an indication from the direct current voltage output of the FM detector. Y

While I have indicated and described a syste for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but `that many modifications may be made without departing from the scope of my invention.

What I claim is:

l. In combination with the locked-in oscillator of a frequency modulation receiver, said oscillator continuously producing oscillations of a predetermined center frequency in the absence of received signals, an electronic indicator tube of the dual shadow type, means responsive to variations in center frequency of the lock-in oscillations for controlling one of the shadow areas of said indi'- cator tube, and means responsive to the magnitude of received. signals for controlling the second shadow area of said indicator tube.

2. In combination with the locked-in oscillator of a frequency modulation receiver, said oscillator continuously producing oscillations of a predeterminedv center frequency in the absence of received signals, an electronic indicator tube of the dual shadow type, means responsive to variations in lcenter frequency of the locked-ln oscillations .for controlling one of the shadow areas of said indicator tube, means responsive tothe magf nitude ci received signals for controlling the sec'- ond shadow area of said indicator tube, both of said control means being so constructed and arranged that said two shadow areas are of mini mum width when the center frequency of received signals is equal to the predetermined center frequency of the locked-in oscill-ations.

3. In a receiver of angle modulated carrier waves of the type including a locked-in oscillator having an input network upon which said carrier waves are applied and an output network feeding -a detector whose characteristic is a sloping filter with spaced response peaks` substantially exceeding thelock-in range of said locked-in oscillator, said oscillator producing oscillations of a predetermined center frequency in the absence of received carrier waves, means responsive to the locked-in oscillator output network for indicating correct oscillator Icenter frequency, and additional means responsive :tothe received angle modulated carrier waves for indicating maximum carrier magnitude.

4. In a receiver of angle modulated carrier waves of the type includ-ing a locked-in oscillator having an input network upon which said carrier waves are applied, and an output network feeding a detector whose characteristic is a sloping filter with spaced response peaks substantially exceeding the lock-in range of said locked-in oscillator, said oscillator producing oscillations of a predetermined center frequency in the absence of received carrier waves, means responsive to the locked-in oscillator output network for indicating L correct oscillator center frequency, additional means responsive to the received angle modulated carrier waves for indicating max-imum carrier magnitude, an electronic indicator tube of the fluorescent target type, a pair of spaced electron deflection rods, and said rods functioning as the respective responsive me-ans.

5. In combination, a source of frequency modulation signals, =a frequency modulation detector having a sloping characteristic between a pair of A peak frequencies, said lcharacteristic having a predetermined center frequency located at the middle of said sloping characteristic, means responsive to said source of frequency modulation signals for positively restricting signal frequency deviations at lthe detector input terminals to a frequency range substantially less than the frequency range between said peak frequencies whereby detection beyond the limits of said peak frequencies is effectively prevented, and said responsive means consisting of a locked-in oscillator.

6. In a one-spot tuning system for a frequency modulation receiver, a frequency responsive detector having a detection characteristic provided with spaced response peak frequencies centered about a pr-edetermined center frequency, said peak frequencies differing by substantially more than the maximum modulation deviations of received signals, a signal transmission network preceding said detector and having output terminals coupled to the detector input terminals, said network having a frequency response characteristic which is centered on said center frequency, said response characteristic .being completely within, and of negligible amplitude at and beyond, the aforesaid peak frequencies.

7. In a system as defined in cl-aim 6, said signal transmission work consisting of a locked-in oscillator, said oscillator having' an input circuit upon which the frequency modulation signals are applied, and said oscillator having a restricted lockin range equal to the band width of said frequency response characteristic.

8. In a receiver of frequency modulated carrier waves of the type inclu-ding a locked-in oscillator having an input network upon which carrier wav-es are applied and an output network, a detector whose characteristic is a sloping filter with spaced response peaks substantially exceeding the lock-in range of said locked-in oscillator, said output network being coupled to the detector, selective means relatively loosely coupled to the locked-in oscillator output network, means connected to the selective means for indicating oscillator frequency variations from a predetermined normal frequency, said oscillator normally producing oscillations of said normal frequencies in the absence of received waves, and additional means responsive to vthe received modulated carrier waves for indieating maximum carrier magnitude.

9. In a receiver of frequency modulated carrier waves of the type including a locked-in oscillator having an input network upon which carrier waves are applied and an output network, a detector whose characteristic is a sloping lter with spaced response peaks substantially exceeding the lock-in range of said locked-in oscillator, said output network being coupled to said detector, selective means relatively loosely coupled to said lock-ed-in oscillator output network, said oscillator normally producing oscillations of a predetermined normal frequency in the absence of received waves, additional means responsive to the magnitude of the received modulated Vcarrier waves, and an electronic indicator 4tube of the fluorescent target type provided with a pair of spaced electron deflection rods, one of the rods being connected to said selective means for indicating oscillator frequency variations from said predetermined normal frequency, and the other rod being connected to said additional means for indicating maximum carrier magnitude.

BENJAMIN S. VlLKOMERSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,165,799 Kimball et al July 11, 1939 2,175,700 Roberts Oct. 10, 1939 2,296,089 Crosby Sept. 15, 1942 2,353,468 Holst et al July 11, 1944 2,395,725 Crosby Feb. 26, 1946 

